Aircraft structural material and method of making the same



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.5. c. M KINNON ETAL AIRCRAFT STRUCTURAL MATERIAL AND METHOD OF MAKING THE SAME Filed Sept. 27, 1943 StAHLJH Wow:

Patented o... 14, 1948 2,455,005

-UNITED STATES PATENT OFFICE AIRCRAFT STRUCTURAL MATERIAL AND METHOD OF MAKING THE SAME John C. MacKinnon, Wellesley, and Walter R. Hickler, Winthrop, Mass., assignors to The B. F. Goodrich Company, New York, N. Y., a corporation of New York Application September 27, 1943, Serial No. 504,016

7 Claims. (01. 154-132) l 2 This invention relates to composite products tures, it will be preferable to use a thermo-setcomprising wood veneer material, fibers, and ting resin such as t phenol-formaldeh de resin molded resins, and to methods of making such sold as Resinox Nomi!- products, and is particularly concerned with the is exceedingly tough and highly water resistant.

production of high strength, low weight mate- 5 Other phenol-aldehyde resins may be used and rials especially adapted for use in certain aircraft urea-aldehyde resins also are quite satisfactory structures and for other purposes. in applications where their somewhat lower water In producing our composite products, we utilize resistance will not be detrimental. The melamine three basic materials: thin wood veneer sheets, resins likewise will be found satisfactory in many fiber sheets, and heat-moldable resins. instances. All such resins may be suspended in The wood veneer material preferably utilized in water. alcohol, or other appropriate solvent or our invention is not the usual relatively thick dispersion medium for application to the fibers sheet material employedin conventional plywood and wood veneer material as hereinafter destructures. According to Perry's "Modern Plyscribed. wood (1942), the customary thickness of wood In a g the composite product of this invenveneer material is from /40" to A" (page 98). tion the tenuous cotton web is impregnated with In contrast to'this relatively great thickness, we the resin in liquid form, the excess being removed preferably utilize wood veneer sheets only .01" by pressing, and then dried. Extensive experithick, this being the thinnest sheet obtainable mentation has shown that best results are atcommercially (Perry, page 106), variations up to tained when about an equal weight of resin is as- .02" being contemplated, however, by references sociated with the cotton web so that the resinherein to wood veneer material of the order of p ated web desirably hould weigh not more .01" thick. Any wood material obtainable in th n 2 u s p square y although i is p s h very thi h t form may b ed, African missible in some cases to increase the total weight mahogany is especially satisfactory and spruce 0 the impregnated Web to as ch s 2% ounces also gives good results, per square yard. Best results are secured when a The fibrous material utilized in the invention b s eb Weig g about 2 ounce per Square preferably comprises any suitable strength-givyard is evenly impregnated with about /2 ounce ing fibers arranged in substantially parallel unof resin to the square yard to produce a total woven relation in tenuous web form. It is posweight of about 1 ounce per Squ e Y in the sible to use fibers of cotton, wool,fiax,ra1nie, pineimp ed Sheet. apple and the like, and even silk fibers and rayon Next, a thin film coating of the same resin or or nylon fiock, although some difliculty may be one cohesively compatible therewith is app to experienced in handling the silk and especially one face of each of two sheets of thin wood veneer the artificial silk fibers because of their smooth 3 material. t o tin being dried as in the case surface characteristics. Because of its cheapness, of the cotton Web.

ready availability, and twisted fibrous structure, several plies 0f the r p a d tenucotton is most satisfactory and is generally to be 0118 b t en are p ed up and-the two wood vepreferred. neer sheets are placed on the outer faces of the The cotton fibers may be arranged in tenuous 40 plied assembly with the resin-treated faces of the web form in any known manner. Preferably, Wood in contact with the resin-impregnated tenhowever, they are carded and then drawn and 110118 e It is 0 po b sp cially n ma spread sidewise to produce an extremely tenuous ins hicker hee ma ri o n rt on or mor web in which the fibers are laid in parallel relaplies of wood veneer material between the cotton tion. For use in the present invention, such webs, the wood inthis case preferably being coatdrawing and spreading preferably should be coned on both faces a d t r y Saturated th tinued until the fibrous web weighs not more than the resin. from about 0.3 to about 1 ounce per square yard. In either case the assembled structure is sub- For best all-round results, the tenuity of the sheet jected to molding heat and pressure to integrate should be substantially that represented by a the structure and set the resin. The assembly weight of about ounce per square yard. should be molded under sufiloient pressure to The resin utilized may be any appropriate heatforce the resin substantially into the wood strucmoldable resin, including both thermo-plastic and ture of the face sheets but the pressure should thermo-setting types of resins. For most purnot 'be suflicient'to cause the resin to flow enposes. however, and especially for aircraft structlrely through the wood. Consequently, the outer faces of the wood veneer sheets remain free of resin in the finished products.

In ajypical case utilizing Resinox No. 790 resin and '.0'1"'African mahogany wood veneer, the assembly may be subjected to a pressure of about 60 lbs. per square inch and a temperature of approximately 300 F. for 20 minutes to bring about the desired results. Using these and similar materials, the pressure should not in any case be greater than about 75 lbs. per square inch, this being substantially lower than conventional molding pressures which normally range from 300 lbs. to 1,000 lbs. per square inch.

After molding, the material preferably should be allowed to cool in the press for some 15 minutes after which the pressure may be released and the finished sheet material removed from the press.

In the present products the fibrous webs are utilized in a quantity suflicient to impart to the structure tensile strength in the aggregate usually exceeding and always at least substantially equalling the tensile strength imparted by the wood veneer material in the aggregate.

It will be understood that the various piles of cotton fiber and of wood veneer material may be assembled with all the grains running in the same direction in order to concentrate the strength in that direction or some or all the plies of these materials may be oriented at the same the core section may be varied widely. In making large sheets, the plies both of cotton and of wood may be lapped at the joints, the resulting irregularity being adequately taken up during the molding operation.

The accompanying drawings illustrate typical applications of the principles hereinabove described.

Of the drawings,

Fig. 1 is a sectional elevation showing an aircraft wing structure of the tension field type embodying the material of the present invention in the web or bulk-head member.

Fig. 2 is a vertical section taken on line 2-2 of Fig. 1.

Fig. 3 is a fragmentary perspective view partially broken away and sectioned for clarity of illustration and showing a typical material of the present invention in a preliminary state, the constituent plies being assembled ready for the heatmolding operation.

Fig. 4 is a similar view illustrating a modified structure embodying the invention.

Fig. 5 is a fragmentary perspective view showing a stiffened structural member embodying material of the invention molded in irregular hatsection form in combination with plain sheet material embodying the invention.

Fig. 1 illustrates a typical use of the present material in the so-called "tension-field" type of aircraft wing structure. Such a structure usually comprises a leading spar I, and a trailing spar 8, forming the backbone of the wing structure. About this backbone is fashioned a skin surface 9 contoured to function as a supporting air foil. The structure is strengthened at intervals along the length of the wing by interior webs on bulkhead-like structures which aid in resisting the crushing load of air pressures during flight, the term tension field web material having reference to these bulkhead-like members.

Thus, in the structure illustrated, a web sheet of the present material I0 cut to appropriate shape and provided with the usual apertures for inspection purposes is mounted somewhat like a bulkhead inside the wing structure, the edges of the material being glued to the skin surface 9 by means of any appropriate wood adhesive and the joint being reinforced by cap strips l|-ll or the like glued to the wing structure and to the web material l0. Hat sections 12, I2, hereinafter more fully described may be provided where required by stress conditions.

Appropriate material I0 for such tension field web use, as illustrated in Fig. 3, may comprise seven core plies Ill-l3 of tenuous cotton web material and two surface plies of thin wood veneer material l4-l4, the several plies being treated and molded in the manner hereinabove described in greater detail. Using .01 wood veneer and cotton webs weighing about A; ounce per square yard, such a combination of materials will produce a final molded product about 0.03" thick. Notwithstanding its thinness, this material has suflicient strength to perform effectively as the tension field web of aircraft wings.

An alternative structure Ilia, especially adapted for somewhat thicker composite materials, is shown in Fig. 4 and comprises a central core ply l5 of thin wood veneer material thoroughly impregnated with resin and interposed between tenuous cotton web plies l3l3 disposed 5 plies on either side of the central wood ply, together with single thin wood veneer plies H-ll on each outer face.

In general, tension field web material will include from 5 to 15 piles of cotton web material in comunction with two or more surface piles of wood veneer material. Substantially greater thicknesses of material may be built up where required by multiplying the number of plies of either or both the cotton and the wood veneer material.

The composite material it] or l0a may be molded in irregular shapes such as the hat section l2 shown in Fig. 5, the hat section being glued to a flat sheet I0 of similar material, or to other appropriate material, to form a stiffened sheet structure for use in aircraft as illustrated in Fig. 1. Such hat sections are molded in a variety of shapes and contours.

Authough especially useful in the tension field type of aircraft wing structures, the present materials are finding wide application in other portions of aircraft and particularly as hat sections, flooring, and bearing plates. It will likewise be understood that the material is useful in a wide variety of other applications outside the aircraft field, including vehicles used in other forms of transportation, as well as articles for personal use such as luggage and for interior finishes in buildings.

Numerous modifications and variations may be effected in details of the procedures, materials, and structures hereinabove described without departing from the spirit and scope of the invention. For example, one of the face sheets of wood veneer material may be replaced by other types of material or omitted entirely. An especially promising adaptation of the invention resides in retaining one face of the sheet free of resin as described, while saturating the opposite face,. whether with or without veneer, and molding it with a knurled or non-skid impression to provide a non-skid surface useful as flooring. In such structures, as well as in certain other modified etlin'tri iiiluwi structures, the wood veneer sheets will be somewhat thicker than the preferred extremely thin sheets hgreina'bove described. The invention accordingly is not to be limited except as specifically indicated in the appended claims.

We claim:

1. A "heat-molded composite product comprising a core section including a plurality of tenuous fiber plies each comprising unwoven fibers arranged in parallel relationship and in such tenuity as to constitute an unimpregnated fibrous sheet weighing not more than about one ounce per square yard; a surface layer comprising a sheet of wood veneer material of the order of .01 inch thick; and a monolithic mass of thermo-set resin bonding the fibers and wood into a unitary integral structure, the resin mass permeating and enveloping completely the entire core section including the tenuous fiber webs but extending only partially into the structure of the wood veneer surface layer, the outer exposed face of the wood veneer being free of the resin, the unwoven fiber webs being present in sufiicient quantity to impart to the composite structure tensile strength in the aggregate at least substantially equalling that imparted by the wood veneer material in the aggregate.

2. A heat-molded composite product comprising a core section including a plurality of tenuous fiber plies each comprising unwoven cotton fibers arranged in parallel relationship and in such tenuity as to constitute an unimpregnated fibrous -sheet weighing not more than about one ounce per square yard; a surface layer on each face of the core section, each comprising a sheet of wood veneer material of the order of .01 inch thick; and a monolithic mass of heat-molded resin bonding the fibers and wood into a unitary integral structure, the resin mass permeating and enveloping completely the entire core section including the tenuous fiber webs but extending only partially into the structure of the wood veneer surface layers, the outer exposed faces of the wood veneer being free of the resin, the unwoven fiber webs being present in sufficient quantity to impart to the composite structure tensile strength in the aggregate at least substantially equalling that imparted by the wood veneer material in the aggregate.

3.A heat-molded composite product comprising a core section including at least one ply of wood veneer material and a plurality of tenuous fiber plies each comprising unwoven cotton fibers arranged in parallel relation ship and in such tenuity as to constitute an unirii'pregnated fibrous sheet weighing not more than about one ounce per square yard, the wood veneer ply being interposed between plies of the tenuous fiber webs; a surface layer comprising a sheet of wood veneer material of the order of .01 inch thick; and a monolithic mass of heat-molded resin bonding the fibers and wood in To a unitary integral structure, the resin mass grmeating and enveloping completely all the several plies comprising the core section but extending only partially into the structure of the wood veneer surface layer, the outer exposed face of the wood veneer being free of the resin, the unwoven fiber webs being present in sufiicient quantity to impart to the composite structure tensile strength in the aggregate at least substantially equalling that imparted by the wood veneer material in the aggregate.

4. Aircraft tension-field web material of the order of a few hundredths of an inch thick and comprising a heat-molded sheet product comprising a core section containing from 5 to 15 piles of tenuous fiber webs each comprising cotton fibers arranged in such tenuity as to constitute an unimpregnated web weighing not more than about one ounce per square yard; surface layers each comprising a single sheet of wood veneer material of the order of .01 inch thick and a monolithic mass of heat-molded resin bonding the fibers and wood into a unitary integral structure, the resin mass permeating and enveloping completely the tenuous fiber webs but extending only partially into the structure of the wood veneer surface layers, the outer exposed faces of the wood veneer being free of the resin, the unwoven fiber webs being present in suflicient quantity to impart to the composite structure tensile strength in the aggregate at least substantially equalling that imparted by the wood veneer material in the aggregate.

5. Aircraft tension-field web material" of the order of a few hundredths of an inch thick and comprising a heat-molded sheet product comprising a core section containing at least one ply of wood veneer material and from 5 to 15 plies of tenuous fiber webs each comprising cotton fibers arranged in such tenuity as to constitute an unimpregnated web weighing not more than about one ounce per square yard; surface layers each comprising a single sheet of wood veneer material of the order of .01 inch thick; and a monolithic mass of heat-molded resin bonding the fibers and wood into a unitary integral structure, the resin mass permeating and enveloping completely the wood veneer and tenuous fiber webs in the core but extending only partially into the structure of the wood veneer surface layers, the outer exposed faces of the wood veneer being free of the resin, the unwoven fiber webs being present in sufiicient quantity to impart to the composite structure tensile strength in the aggregate at least substantially equalling that imparted by the wood veneer material in the aggregate.

6. Structural material comprising a base member in sheet form and a stiffening member adhered thereto, at least one of said members comprising a heat-molded composite product comprising a core section including a plurality of plies comprising unwoven cotton fibers arranged in substantial parallelism in tepggus web form, a surface layer comprising a sheet of wood veneer material of the order of .01 inch thick, and a monolithic mass of heat-molded resin bonding the fibers and wood into a unitary integral structure, the resin mass permeating and enveloping completely the tenuous fiber webs but extending only partially into the structure of the wood veneer surface layer, the outer exposed face of the wood veneer being free of the resin, the unwoven fiber webs being present in sufiicient quantity to impart to the composite structure tensile strength in the aggregate at least substantially equalling that imparted by the wood veneer material in the aggregate.

7. A method of making composite products W which comprises carding cotton fibers, drawing and spreading the carded fibers to produce a tenuous web weighing not more than one ounce per square yard and in which the fibers are arranged in substantially parallel relation, treating the tenuous web with a thermo-setting resin, arranging several plies of the treated web in superposed relation together with a surface layer of wood veneer material of the order of .01 inch thick, and subjecting the assembly to heat and pressure to integrate the structure and set the resin, the molding pressure being not greater thangbouj; 75 lbs, per square inch so that the reslfiis 'forced substantially into the wood veneer structure but not entirely through it, the exposed face of the wood being substantially free of resin when the molding operation is completed.

JOHN C. MACKINNON.

.WALTER R. HICKLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,299,747 McClain Apr. 8, 1919 1,318,742 Frederick Oct. 14, 1919 OTHER REFERENCES Modern Plastics, May 1943, pages 100, 130 and 131, 154 2CW.

Certificate of Correction Patent No. 2,456,005. December-i,i1 9 4 8.

JOHN C. MACKINNON ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 71, for the words webs on read webs or; column 4, line 5], for

Authough read Although; line 52, for type read types; column 6, line 7, after the Word thick insert a semi-colon; column 7, line 3, after 75 lbs. strike out the comma;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 22nd day of November, A. D. 1949.

THOMAS F. MURPHY,

Assistant Gammz'ssioner of Patents. 

